A widely used method in the prior art for the manufacture of silicon single crystals is the so-called Czochralski method (CZ method). This CZ method comprises a step for the melting of polycrystalline silicon in a crucible manufactured from quartz glass, a step for the immersion of a “seed” crystal for the silicon single crystals in this silicon melt, and a step for the rotation of the crucible as the “seed” crystal is gradually pulled up which, with the “seed” crystal serving as a core, results in the growth of silicon single crystals. The silicon single crystals produced by the abovementioned CZ method must be of high purity and must afford the manufacture of a good yield of silicon wafers and, accordingly, generally employed quartz glass crucibles for the manufacture of said comprise a two-layered quartz glass crucible structure configured from a non-foamed transparent inner layer and a foamed non-transparent layer.
Because of the increased time required for the pulling up of silicon single crystals accompanying the increase in the aperture diameter of silicon single crystals that has occurred in recent years, further and increased purity of the quartz glass crucible has come to be demanded. With this in mind, the applicants of this application have previously proposed a crucible of a two-layered quartz glass crucible structure configured from a transparent inner layer and a non-transparent outer layer of which the inner layer is formed from synthetic silica powder (cited document 1). Because of the very low quantity of impurities contained in the inner layer configured from the synthetic quartz glass and, in addition, because there is minimal roughening and formation of cristoballite spots on the crucible surface accompanying the pulling up of the silicon single crystals, this crucible affords an improved yield of pulled up silicon single crystals. This crucible is additionally advantageous in that, because the abovementioned outer layer of the crucible is configured from a high viscosity natural quartz glass of high Al concentration, the likelihood that the crucible will break or buckle or that similar deformation will occur at high heat load can be reduced.
However, there are problems inherent to the use of a crucible configured as a two-layered structure such as that described above in which the transparent inner layer is configured from synthetic quartz glass and the outer layer is configured from natural quartz glass in that, because of a resultant significant increase in heat load produced by the heater that has accompanied the increase in the aperture of the crucible, at times of long contact with the silicon melt and high heat load in particular, warp is generated at the interface between the two layers in the curved portion of a long crucible due to the difference in viscosity that has its origin in differences in Al concentration and, accordingly, warp deformation of the transparent inner layer occurs, large undulations are formed in the crucible inner surface, and yield of the silicon single crystals is reduced.
Meanwhile, although oxygen is present in the silicon single crystals produced using the CZ method, the concentration of this oxygen, which is dependent on the dissolution of the quartz glass crucible in the melt, is governed by the difference between the quantity of the quartz glass crucible (SiO2) that dissolves and the quantity of SiO that evaporates from the surface of the silicon melt. In general, despite the fact that the surface area of the surface of the melt remains essentially unchanged as the pulling up of the single crystal progresses, because the contact surface area with the quartz glass crucible decreases and, accordingly, the dissolved quantity thereof decreases, an oxygen concentration gradient is produced in the length direction of the single crystal. Even if the pulling up of the silicon single crystals is implemented without interference, as this gradient increases an increase in the section in which the oxygen concentration is outside the standard range occurs which produces the undesirable effect of lowered product yield. Although this problem has been addressed in the prior art by increasing the quantity of the quartz glass crucible that dissolves by increasing the number of rotations thereof and, in addition, by raising the pressure in the furnace to decrease the quantity of SiO evaporated from the surface of the melt, the inherent drawback associated with these measures is the increased manufacturing costs caused by the increased complexity of the operations. Although, in order to obviate this drawback, a quartz glass crucible comprising a semi-circular base part in which balance is maintained between the surface area of the melt and the contact surface area between the crucible and the melt has been proposed (cited document 2), there is an inherent drawback associated with this quartz glass crucible in that, because its shape differs from the shape of the flat-base quartz glass crucible of the prior art, significant alterations must be made to the equipment used for the manufacture thereof which results in a substantial increase in manufacturing costs.                Cited document 1: Japanese Patent No. 2811290, Japanese Patent No 2933404        Cited document 2: Japanese Laid-Open Patent Application No. H11-199368        